The basic requirement of launching a space vehicle is achieving the required speed to escape earth’s gravitational field and remain in orbit. The speed required? Well Mach 24 should pretty much do it. Once the craft is in orbit it essentially is falling fast enough to continue escaping, yet being pulled back to earth resulting in an angular trajectory that keeps it a fixed radial distance from the earth. You could argue that space vehicles in earth orbit are not floating but rather falling continuously needing to balance the “escape vector (centrifugal force)” with the “gravitational vector” meaning that in all intense and purposes the vehicle is still subjected to gravity but that the continuous fall is similar to the zero gravity experienced in zero g parabolic flight within our own atmosphere.
The video is adopted from youTube and offers a simple and brief explanation towards the workings of spacecraft.
Now to achieve a speed of Mach 24 is not the biggest problem faced, rather it is having enough fuel for a long enough and sustained rocket burn to achieve earth orbit. Up to what height/distance does the vehicle need to go to allow for a zero burn trajectory? Well, we need to escape the smallest of air particles which would induce drag on the craft’s frame, slowing it down and thus allowing gravity to exceed centrifugal force and pulling it back to earth. This would mean at least 160 kilometres from the earth’s surface where the atmosphere has become so remote or so thin that drag is no longer a factor. 160 kilometres would be known as “low earth orbit” and spacecraft generally need much higher altitudes due to combinations of weight, size, mission requirement and other factors that combine to decide on the required parameters. More altitude is equal to more thrust which means more fuel. More fuel is more weight which means more power. Fortunately the power available versus power required curve of a rocket engine which relies on self contained fuel, is one which though demanding, allows the boundaries of such flights to be overcome. As the craft gets further from the earth the weight effectively reduces and thus so does the power required and thrust required but the secret is in overcoming the massive requirement where gravity is at its strongest. It is here that you need to have sufficient thrust to not only overcome gravity but to do so while carrying enough reserves of fuel for sustained burn all the way to your target altitude.
Simply getting into space/ low earth orbit is not the challenge it is going higher as you even have to carry the oxygen required for combustion. The reason thus for an as straight-up path and as little time as possible with as much thrust as possible should thus be quite evident: less distance to cover = less burn time needed = less fuel to be carried = less weight = more thrust available. In the atmosphere, aircraft are continually fighting the resistance of air particles and thus experience the ever present drag which apart from literally holding the craft from moving forward, also sees heat being generated due to the friction between the craft’s skin and the air particles. This is the primary limit of atmospheric craft in terms of speed. An example would the the legendary SR71 Blackbird. After the first flights pilots would report that even at high altitude (approximately 80000 feet), at Mach 3+, the SR71’s skin would glow due to the heat generated from friction. Spacecraft, need instead to push through this limit as fast as possible to generate the escape velocity of Mach 24 required to set-up the centrifugal force needed to balance gravity in a constant resultant fall. Now, the quicker you push through the atmosphere, the less fuel you need but this means massive thrust required out of as little fuel as possible. To generate this massive thrust, spacecraft require ultimately more fuel in comparison to aircraft with as much as 85% of their weight consisting out of fuel on launch while as an example, a very capable modern fighter would sport a fuel fraction of around 0.25 meaning that only 25% of its total mission weight might consist of fuel.
Now looking at the issues mentioned, it should be clear that the most important thing to do is to reduce the reduce the weight and the fuel required by shortening burn time and reducing the thrust required for an improved launch situation. There are factors at play which greatly influence this such as: latitude, launch direction, and launch angle amongst others.
Latitude: Launching near or at the equator helps substantially due to the oblate spheroidal shape of the earth as well as the manner in which the atmosphere interacts with the planet. Basically, a spacecraft launched from Central Canada would require at least 10-12% more thrust to reach orbit than one from the Equatorial Regions.
Launch Direction: Earth’s rotation, when considered in terms of gravity and the interaction of the atmosphere with a frictional rotating earth, means an “increase” in velocity resulting in around 5% less thrust versus launching towards the West as the time through the atmosphere will be reduced due to a shorter distance to fly through the atmosphere versus the reverse direction. Kind of hard to understand right away but imagine that the road on which you are travelling suddenly moves in a direction which assists your cause (like the airport walkways) while you retain a constant speed relative to the road. Your distance covered and thus apparent speed in relation to a stationary reference in space has increased dramatically.
Launch Angle: Launching at 45 degrees would require about 30% more thrust to reach orbit than launching from an ideal angle of 70-80 degrees because of the longer path travelled through the denser part of the atmosphere. If the launch were horizontal, the speed gained in launch would be lost as energy would be bled-off during the long upward turn required through the dense lower atmosphere in order to escape the atmosphere. In this turn g loading also becomes very critical as even a turn at 7g in the denser atmosphere would require a good 9 to 10 kilometres radius to complete a 90 degree turn. A “g” force of 7 can be easily achieved with a speed of Mach 0.5 and we need to get to Mach 24…get the picture? Squashed astronauts and popping fuel tanks with an incredible amount of distance to cover with a horizontal launch in today’s launch technology.
Now why can Virgin Galactic hang a spaceship under an aircraft and briefly launch people into space? It is easy to answer if you think of the above dilemmas: the Virgin Spaceship carries no equipment, no extra weight and merely pops into low earth orbit falling directly back to earth.
It is hoped an dreamed that future technology will allow propulsion systems that would allow the concept of the Virgin programme or better, of a horizontal take-off, fly out into space and return concept to be met. This means however totally new propulsion systems capable of generating incredible amounts of thrust for sustained periods of time which would allow a craft sufficient endurance and time to climb to altitudes required without the drama of excessive g forces flattening anything in their path.
Engenya remains committed to resolving future problems today for a better tomorrow.
A layman’s brief and straight-forward explanation of Space Launch Dynamics by Engenya GmbH